1. Technical Field
The present invention generally relates to a sliding member such as valve lifter in an internal combustion engine, and a method of manufacturing a sliding member.
2. Description of Related Art
Japanese Laid-Open Utility Model Publication H4-121404 discloses a valve lifter having a shim that slideably contacts a cam for driving an intake/exhaust valve of an internal combustion engine. The shim of the valve lifter needs to have a sliding surface whose surface roughness is sufficiently small to minimize friction. At the same time, the sliding surface has to have a sufficient hardness in order to prevent excessive wear in the sliding surface, and also to prevent an increase in friction due to the increase in surface roughness of the sliding surface from the wear.
Therefore, it has been known to smooth the base metal of the sliding member, such as a valve lifter, with high precision through a lapping process, and thereafter to create a hard material protection coating such as a titanium nitride through physical vapor deposition (PVD) on its top sliding surface.
There exists a need for a sliding member that has low friction and superior durability at a low cost in comparison of the above mentioned prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
It has been discovered that when a hard material coating is created on a sliding surface through physical vapor deposition, it is necessary to perform the process using a vacuum furnace. Accordingly, only a limited number of pieces can be processed at a time. Therefore, manufacturing cost of the sliding member or cam follower becomes very expensive. The present invention has been conceived in view of the aforementioned problem.
One of the objects of the present invention is to provide a sliding member that has low friction and superior durability at a low cost.
In accordance with one aspect of the present invention, a sliding member is produced that comprises a base metal, a diffusion layer, and a compound layer. The diffusion layer has a first predetermined depth and overlies the base metal. The compound layer has a second predetermined depth and overlies the diffusion layer. The diffusion layer and the compound layer are formed on the base metal through a nitriding process. The second predetermined depth of the compound layer is formed by a polishing process on an outermost layer portion of the compound layer such that an original depth of the compound layer formed by the nitriding process is reduced in depth to the second predetermined depth of the compound layer so that a smooth top sliding surface remains.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.